What is vitamin D deficiency? How less sun can make you vulnerable to infection

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insider@insider.com (Madeline Kennedy)

• The primary cause of vitamin D deficiency is lack of sunlight exposure.
• Vitamin D helps keep your immune system strong, so a deficiency could be the reason for frequent colds or the flu.
• You can get more vitamin D by spending at least 5 to 10 minutes outside 3 times a week without sunscreen. 
• This article was medically reviewed by Melissa Rifkin, MS, RD, CDN, owner of Melissa Rifkin Nutrition LLC. 
• Visit Insider's Health Reference library for more advice.

Vitamin D is an essential vitamin that plays a crucial role in the body, from maintaining bone health to strengthening the immune system. 

You can get vitamin D three ways: through foods you eat, from dietary supplements, and by natural synthesis in the body through exposure from the sun. Yet, an estimated 40% of US adults don't get enough of the so-called "sunshine vitamin".

When the body doesn't have enough vitamin D to stay healthy, it's referred to as vitamin D deficiency. And surprisingly, it's more common than you might think. 

Causes of vitamin D deficiency

Unlike other nutrients, vitamin D is produced by the body naturally when the skin is exposed to the ultraviolet rays from the sun. Depending on where you call home, you should try to get at least 15 minutes of sunlight between peak intensity hours from 10 a.m. and 3 p.m., three times a week, in order to get enough vitamin D. 

In addition to sunlight, you can obtain vitamin D naturally through foods like salmon, mushrooms, and eggs. But because the foods that contain vitamin D are limited, some everyday staple items like milk, yogurt, orange juice, and cereal are fortified with vitamin D. 

Vitamin D supplements are another way to ensure you're getting your daily dose. Supplements can be advantageous for all healthy adults, but also if dietary intake is not possible (due to malabsorption) or exposure to sunlight might be dangerous (for those with sensitive skin).

You can become vitamin D deficient for several reasons:

• Lack of sunlight exposure
• Not eating enough vitamin D-rich foods
• Being unable to absorb or metabolize vitamin D

Risk factors

Research has shown that certain groups of people and some with particular medical conditions are at a higher risk of inadequate vitamin D levels:

Skin tones: Because people with dark skin have more of the pigment called melanin — which reduces the skin's ability to absorb UVB rays — many scientists conclude that those with darker skin tones are more likely to be at risk of vitamin D deficiency. 

Conversely, light-skin people, especially those with a family history of skin cancer, may inhibit absorption by applying a lot of sunscreen, or avoid the sun altogether to protect their skin from damage. 

Older adults: Aging adults face a range of potential causes, including changes in the skin, reduced sunlight exposure from spending more time indoors, and the inability of the body to convert vitamin D.

Infants: Breast milk alone is not enough to give infants the adequate amount of vitamin D. According to the American Academy of Pediatrics, infants who are breastfed or partially breastfed should also be given 400 IU of a vitamin D supplement per day beginning from the first few days of life. 

While rare for infants and children, severe vitamin D deficiency can lead to rickets, a condition that causes delayed growth, muscle weakness, and skeletal deformities.

Dietary restrictions: Very small amounts of vitamin D can be found in plants, making those who follow vegan or vegetarian diets more likely to get less vitamin D. Lactose intolerance and gluten-free diets have also been linked to lower levels of vitamin D. 

Obesity: According to the Harvard School of Public Health, people who are obese are more likely to have low vitamin D levels because vitamin D is a fat soluble vitamin, meaning it can gather in excess fat tissue, which makes it harder to be released.  

Moreover, chances of deficiency are much higher for people who have gone through weight loss surgeries that reduce the size of the stomach or alter the intestine, which makes it more difficult to absorb and synthesize nutrients.

Medical conditions: Low vitamin D levels are associated with people with Chron's disease, celiac disease, or cystic fibrosis. These conditions lead to malabsorption of nutrients, including vitamin D. For the same reasons, those with kidney and liver problems are also more likely to be vitamin D deficient.

Geographical factors: Seasonality and where you live could have an impact on your vitamin D intake. For example, people living in the Northern Hemisphere may not get enough vitamin D from sunlight in the winter. 

Other lifestyle factors: Generally speaking, vitamin D deficiency is commonly linked to other factors and behaviors, such as a work environment and its effect on time spent outdoors, or wearing clothing that covers the skin from head to toe. 

Symptoms of vitamin D deficiency

Back pain, bone pain, and muscle weakness are common signs of vitamin D deficiency. Severe deficiency can cause osteomalacia and osteoporosis, conditions where your bones become less dense and more likely to fracture or break, says Kelly Springer, a registered dietitian.

Overall, signs and symptoms are subtle, and may include: 

• Frequent colds or respiratory infections: Vitamin D helps keep your immune system working well, so a deficiency can reduce your chance of fighting off illness. A 2017 study found that a regular dose of vitamin D can even protect you against colds and the flu. 
• Fatigue: In women, vitamin D deficiency has been linked to feeling fatigued.
• Depression: Springer notes that vitamin D has links to depression, but adds that there is still more research needed before doctors recommend vitamin D supplements to treat depression.

Vitamin D dosage

Just how much vitamin D should you be taking daily? The average adult needs around 600 international units (IU) of vitamin D a day. For reference, a serving of salmon contains roughly 400 IU.

Recommended dosage is different across age. The chart below shows the daily intake needed to maintain bone health, according to the Food and Nutrition Board:

© Shayanne Gal/Insider Shayanne Gal/Insider

Moreover, the Mayo Clinic says that you can safely take 1,000 to 2,000 IU per day from supplements. 

How to treat Vitamin D deficiency

One of the most important changes you can make to raise your vitamin D levels is to get more sunlight. "Try to get at least five to 10 minutes outside, three times a week without sunscreen," Springer says. This is most effective when your skin is directly exposed to the sun and not covered by clothing.

If you aren't able to get much sunlight, you can also make dietary changes as well. "During the winter months, try consuming foods that contain vitamin D or taking a supplement," Springer says.

You can also boost your vitamin D using a UV lamp, much like the type used for indoor tanning. This is particularly helpful for people with certain medical conditions, like kidney or liver diseases, who can't absorb vitamin D from supplements.

When choosing a supplement, Springer recommends that most people should aim for 600 IU per day, while older adults are recommended to have 800 IU per day.